Dimerization and/or dehydrogenation of alkanes

ABSTRACT

Barium peroxide oxidizer, together with a transition metal from Group I, III, IV, V, VII or VIII or compound thereof is used as stoichiometric reagent in the oxidative dimerization of hydrocarbons having three or four carbon atoms. Barium peroxide oxidizer, together with a transition metal from Group I, III, IV, V, VI, VII or VIII or compound thereof is used as stoichiometric reagent in the dehydrogenation of hydrocarbons having three or four carbon atoms.

BACKGROUND OF THE INVENTION

In our U.S. Pat. No. 5,073,664 issued Dec. 17, 1991, a process for thecoupling of alkanes at low temperature over a regenerable stoichiometricreagent, barium peroxide, is disclosed and claimed. We have now foundthat by modifying the peroxide by incorporation of transition metalcomplexes or salts, reactivity is significantly altered even thoughlevels a low as 1% (wt) of a transition metal are added to the peroxide.New reactions have also been discovered depending on the choice of theadded metal.

PRIOR ART

The oxidative coupling of methane to give ethane and ethylene has beenwidely studied over the past two decades. Among catalysts for thisprocess are the reducible metal oxides such as PbO, MnO, LiO/MgO andmany others. They are used with or without promoters. Temperaturesrequired for the process usually exceed 650° C. which is well above thetemperature of 450° C. at which the isoparaffin dimers of propane andbutane thermally crack.

In Journal of Catalysis (1990) pp. 121-122, Otsuka et al indicated thatpropane coupling could occur at low yield in a stoichiometric anaerobicreaction over sodium peroxide at 375° C. It is, however, difficult toregenerate the reduced sodium product with molecular oxygen.

SUMMARY OF THE INVENTION

According to our invention, hydrocarbons having three or four carbonatoms in the molecule are oxidatively dimerized by contact in a reactionzone with a reagent comprising barium peroxide oxidizer and an addedtransition metal or compound thereof at a temperature sufficient tooxidatively dimerize the hydrocarbon and reduce the barium peroxide tobarium oxide or other by-product, then a branched chain dimer of thehydrocarbon and reduced reagent comprising largely barium oxide and themetal or compound thereof are recovered from the reaction zone, and thereduced barium species is oxidized to regenerate said reagent.

The reaction temperature in the oxidative dimerization is 200° to 450°C., preferably 250° to 400° C., more preferably 300° to 400° C. Thereaction can be done in liquid or vapor phase, preferably in the vaporphase, at pressures of 0 to 2000 psig, preferably 400 to 1200 psig. Thetemperature will vary depending on the processing arrangement and thefeedstock, but the minimum temperature necessary is easily determined.

In another embodiment of the invention, hydrocarbons having three orfour carbon atoms in the molecule are dehydrogenated by contact in areaction zone with a reagent comprising barium peroxide oxidizer and anadded transition metal or compound thereof at a temperature sufficientto dehydrogenate the hydrocarbon and reduce the barium peroxide tobarium oxide or other species, then dehydrogenated hydrocarbon andreduced reagent comprising largely barium oxide and the added metal orcompound thereof are recovered from the reaction zone, and the reducedbarium species in the reduced reagent is oxidized to regenerate thereagent.

The reaction temperature in this embodiment is 200° to 650° C.,preferably 250° to 450° C., more preferably 300° to 400° C. The reactioncan be done in liquid or vapor phase at pressures of 0 to 2000 psig,preferably 400 to 1200 psig.

It is possible to simultaneously oxidatively dimerize a portion of thehydrocarbon feed and dehydrogenate another portion of the feed. In suchcase, hydrocarbons having three or four carbon atoms in the molecule arecontacted in a reaction zone with a reagent comprising barium peroxideoxidizer and a transition metal or compound thereof at a temperaturesufficient to oxidatively dimerize part of the feed and dehydrogenateanother part of the feed and reduce the barium peroxide to barium oxide,then branched chain dimer, dehydrogenated hydrocarbon, and reducedreagent comprising largely barium oxide and the added metal or compoundthereof are recovered from the reaction zone, and the reduced bariumreagent is oxidized to regenerate the reagent. Temperature and pressurefor this embodiment are generally the same as for the oxidativedimerization embodiment above.

DETAILED DESCRIPTION OF THE INVENTION

Barium peroxide is an article of commerce and readily available. It canbe made by the direct combustion of barium or barium oxide in air oroxygen at 500° to 600° C. Because of this, the lower oxides of bariumformed from the barium peroxide, BaO₂, in the oxidative dimerizationand/or dehydrogenation of this invention are readily regenerable tobarium peroxide.

Barium peroxide may be doped with metal by several techniques.Presynthesized or purchased barium peroxide may be doped with metal byimpregnation with aqueous solutions of metal salts or complexes. Forexample, iron (II) tetrafluoroborate (aqueous), chromic nitrate,manganese (II) nitrate, or polynuclear metal complexes such asheteropoly acids (typically Keggin or Dawson structures), or otherpolynuclear metal species, particularly those containing iron orruthenium and at least one labile ligand may be used. Alternatively,barium plus metal can be coprecipitated followed by treatment withconcentrated hydrogen peroxide or with oxygen at high temperature toform the peroxides. A variable amount of barium carbonate, bariumbicarbonate, or barium oxide may also be present.

EXAMPLES

Experiments were performed in which a pulse of an alkane such asisobutane or propane was passed over the doped peroxides at highpressure and at moderate temperatures. On-line analytical systemsindicated the products to be dehydrogenation, coupling, cracking orcombustion products, as shown in Table 1 infra.

                                      TABLE 1                                     __________________________________________________________________________    SUMMARY OF HIGH PRESSURE, ANAEROBIC PULSE REACTOR EXPERIMENTS:.sup.a          STOICHIMETRIC REACTIONS OF ALKANES WITH PROMOTED BARIUM                       __________________________________________________________________________    PEROXIDES.sup.b                                                                                              Carbon Selectivity                                                                              Coupling                                              Conversion    frag.sup.f                                                                         com- Isomer                                                per   C--C    or   bust Ratio Observa-               H-Accep-         T.sup.n                                                                           P.sup.c,m                                                                         Pulse coup-                                                                             dehyd.sup.e                                                                       cracking                                                                           CO.sup.x                                                                           (i-i-i:t-                                                                           tions/                 Run tor    Feed  (°C.)                                                                      psig                                                                              (mol %)                                                                             ling.sup.d                                                                        (atomic % of conv. C)                                                                       t-t:other).sup.g                                                                    Comments               __________________________________________________________________________    1   CdO.sup.h                                                                            C.sub.3 H.sub.8                                                                     614 1512                                                                              32.6  trace                                                                             17.6                                                                              42.9 24.8       bz, tol obs'd          2                375 1513                                                                              0.8   0   18.0                                                                              28.5 6.3        no aromatics           3   BaO.sub.2                                                                            CH.sub.4                                                                            <501                                                                              814 0     0   0   0    0          O.sub.2 obs'd          4          C.sub.3 H.sub.8                                                                     398 800  ca6  33  some     0    1:1.36:0.08.sup.i                                                                   H.sub.2 O,                                                                    C.sub.4, C.sub.3       5          iC.sub.4 H.sub.10                                                                   400 810 low   low ?   ?    ?                                 6   1% Fe/BaO2                                                                           CH.sub.4                                                                            ≦501                                                                       815       low                     C.sub.2 obs'd at                                                              T > 475°        7          iC.sub.4 H.sub.10                                                                   313 815 4.9   62.0                                                                              31.5                                                                              trace                                                                              0          trace O.sub.2                                                                 C.sub.3 's,                                                                   trCH.sub.4             8                343     20.2  72.9                                                                              22.4                                                                              <4.sup.k                                                                           0    1:1.82:.sup.1                                                                       (1st pulse)                       .sup.j                                                                              346     12.0  73.7                                                                              20.8                                                                              .sup.k                                                                             0    1.14:0.19                                                                           (2nd pulse)                             346     9.3   70.8                                                                              24  .sup.k                                                                             0          (3rd pulse)            9                368     17.7  69.9                                                                              20.8                                                                              .sup.k                                                                             0                                 10               399     16.2  51.2                                                                              34.8                                                                              .sup. k                                                                            0                                 11  1% Cr/Ba.sub.0 2                                                                     iC.sub.4 H.sub.10                                                                   345 818 4.6   0   ca95                                       12  1% Ce/BaO.sub.2                                                                      CH.sub.4                                                                            <500                                                                              815       low                     C.sub.2 obs'd at                                                              T > 430°        13         iC.sub.4 H.sub.10                                                                   349 813 2.9   64.8                                                                              35.2                                       14               398     5.8   60.4                                                                              32.1                                                                              0    0                                 15  1% Ag/BaO.sub.2                                                                      iC.sub.4 H.sub.10                                                                   340 815 1.1   40.4                                                                              59.6     0                                 __________________________________________________________________________    .sup.a Packed bed reactor operated isothermally: 400 microliter pulses of     prevaporized feedstock injected via automatic valve into flowing He           carrier gas (400 ml/min. NTP): granulated oxides (18/35 mesh) mixed 1:1       (v:v) with silica gel diluent: pulse profile and continuous                   component identification with time detected with on-line quadrupole mass      spectrometer operated at low voltage ionization currents                      (12 V typically) coupled to reactor system through an open-split              interface (5 ml/min He): detailed product analysis and quantification         performed by capturing slices of the effluent at various points along the     effluent peak profile by way of a computer timed sampling                     valve which feeds three independent multidimensional GC analysis systems      including a Pd thimble for H.sub.2 analysis, back-flushed packed              bed columns/TCD detector for analysis of fixed gases, and a capillary         GC/mass spectrometer detector for hydrocarbon products.                       Identification of products was accomplished by spiking to establish           retention times of known compounds and by computer-based                      comparision to library mass spectral cracking patterns in a commercial        data base. Typical pulse dosage was 62 gram-atoms oxide                       per mole of hydrocarbon pulse. Samples were pretreated at 375° C.      to remove superoxide impurities and each pulse was followed by a              TPD experiment to look for strongly held absorbates up to 500° C.      typically.                                                                    .sup.b Promoted barium peroxides were prepared by aqueous impregnation of     commercial granulated BaO.sub.2 with ferrous tetrafluoroborate                (or BF.sub.4.sup.-  salts of other metal ions) to appropriate levels          followed by vacuum drying at 90° C. for 12 hours.                      .sup.c 0.101 MPa = 14.7 psi = 1 bar.                                          .sup.d C.sub.8 isomers for I-C.sub.4 H.sub.10 feed runs; C.sub.6 products     for C.sub.3 H.sub.8 feed runs; C.sub.2 products for CH.sub.4 runs;            ethylene glycol for CH.sub.3 OH feed runs.                                    .sup.e Isobutene or propene for isobutane or propane feedstocks               respectively.                                                                 .sup.f For isobutane feed, fragmentation products include propene,            propane, ethylene, ethane, methane; for propane feed, fragmentation           products include ethylene, ethane, methane.                                   .sup.g coupled product isomers include:                                       isobutane feed             propane feed                                       internal-internal: 2,2,3,3,-tetramethylbutane, (3°-3°)                                     2,3-dimethylbutane, (2°-2°)          internal-terminal: 2,2,4-trimethylpentane, (3°-1°)                                         2-methylpentane, (2°-1°)             terminal-terminal: 2,5-dimethylhexane, (1°-1°)                                              -n-hexane, (1°-1°)                  .sup.h CdO was not diluted with silica gel.                                   .sup.i,l Expected ratios of coupling products can be calculated based on      statistical availability of C--H, corrected for relative abstraction          rates for differend C--H types, and corrected for differences in              recombination rates influenced by steric constraints in radical               intermediates which must come together:                                                        STATISTICAL                                                                            AVAILABILITY.sup.p                                                                          LITERATURE.sup.q                              COUPLING AVAILABILITY                                                                           & REL. ABSTR. GAS PHASE  FOUND                              PRODUCT  OF C--H  RATE          ABSTR. & RECOMB.                                                                         THIS                       __________________________________________________________________________                                                       STUDY                      i-C.sub.4 H.sub.10                                                                    3°-3° (i-i)                                                              1        1             1          1                                  3°-1° (i-i)                                                              9        0.12          0.38       1.82                               1°-1° (i-i)                                                              81       0.015         0.046      1.14                               other C.sub.8                                                                          --       --            --         0.19                       __________________________________________________________________________     .sub.-- C.sub.3 H.sub.8                                                              2°-2° (i-i)                                                              1        1             --         1                                  2°-1° (i-i)                                                              3        0.04          --         1.36                               1°-1° (i-i)                                                              9        0.0014        --         0.08                       .sup.j Repetitive pulses over same sample.                                    .sup.k Major fragmentation product was propene; only a trace of CH.sub.4      observed.                                                                     .sup.m ±3 psig typical during run.                                         .sup.n ±2° C. typical during run.                                   .sup.o O.sub.2 desorbed 430-513° C.                                    .sup.p E. W. R. Steacle, Atom and Free Radical Reactions, vol II.             Reinhold Publ., NY, 1954, pp. 500.                                            .sup.q B. deB. Darwent and C. A. Winkler, J. Phys. Chem. (1945) 49.           __________________________________________________________________________    150.                                                                      

Doping of barium peroxide with 1% iron resulted in significantenhancement of dehydrogenation selectivity and overall yield compared toundoped barium peroxide when pulsed with isobutane; no carbon oxides andfew cracking products were observed at 300°-400 degrees C. and about 800psig. In contrast, addition of 1% Cr to BaO₂ resulted in significantenhancement of dehydrogenation selectivity but in no coupling. Ce and Agdoped samples are also shown in Table 1. These results are surprisingand indicate that a process can be tailored to either dehydrogenation orcoupling depending on choice of dopant. C₆ and C₈ branched isomersproduced by this invention are desirable nonaromatic high octanecomponents in motor fuels.

Referring further to Table 1, Runs 1 and 2 show that use of a reducibleoxide such as cadmium oxide resulted in only a trace of coupling ofpropane feed, but significant amounts of dehydrogenation and crackingproducts.

Runs 3, 4 and 5 show that with undoped barium peroxide, 33% selectivityfor coupling of propane is obtained at 398° C. and 800 psig, while withmethane and isobutane at <501° C. and 400° C. respectively, there waslittle or no activity for the undoped barium peroxide.

Runs 6 to 10 inclusive show that barium peroxide doped with 1% iron gavegood selectivity for coupling of isobutane, in contrast to the resultsfor the undoped barium peroxide, as well as substantial activity fordehydrogenation of isobutane. It is expected that the results for theiron-doped barium peroxide would also be much better than for theundoped barium peroxide, with propane a feed.

Run 11 shows that barium peroxide doped with 1% chromium, has highactivity for dehydrogenation of isobutane, but substantially no activityfor coupling of isobutane under the conditions employed.

Runs 12, 13 and 14 show that barium peroxide doped with 1% cerium hasgood activity for the coupling of isobutane and substantial activity forthe dehydrogenation of isobutane as well. It is expected that similarresults would be obtained with other C₃ or C₄ feeds.

Run 15 shows that barium peroxide doped with 1% silver has good activityboth for the coupling of isobutane and for the dehydrogenation ofisobutane. Similar results are expected for other C₃ to C₄ feeds.

As described in the 664 patent supra for undoped barium peroxide, theused metal-doped peroxide reagents can be regenerated with air or oxygenunder appropriate conditions. useful temperatures are 200°-600° C.(depending on the reactivity of the system and stability of the coupledor dehydrogenated hydrocarbons), preferably 300°-400° C. Usefulpressures are 0.1 to 1000 atmospheres, preferably 30-65 atmospheres.

The process according to the invention can be carried out eithercyclically or continuously, as disclosed in the 664 patent supra, thedisclosure of which is hereby incorporated by reference.

If oxygen is introduced into the reaction, oxidative dehydrogenation ispromoted, but the effectiveness of the process for coupling is reduced.

The results of the examples above indicate that although most reduciblemetal oxides and solid "catalysts" for methane coupling are noteffective at generating light alkane radical coupling or dehydrogenationproducts at <500° C., barium peroxide and its derivatives promoted withcertain +2/+3 transition metal salts are efficient reagents for suchconversions at temperatures of 300°-400° C., which are below the thermalcracking range of isodimers. Low level doping of barium peroxide withFe²⁺ (1 wt. %) resulted in enhanced alkane coupling selectivity; whereasdoping with Cr³⁺ resulted in enhanced dehydrogenation selectivity inpulse experiments.

Examination of isomer distributions among isobutane or propane couplingproducts from pulse experiments indicated the predominance ofinternal-terminal coupled products (2,2,4-trimethylpentane or2-methylpentane respectively). This finding is in contrast to literaturereports of the competitive gas phase carbon fragment formation andradical coupling rates which result in the internal-internal products(2,2,3,3-tetramethylbutane or 2,3-dimethylbutane respectively) to bepredominant products in the absence of a surface interaction. Theobserved isomer distributions from iron doped peroxide were also incontrast to our results measured in fluid benzene solution in which the2°-2° coupling product is formed nearly exclusively from propanereacting in the presence of t-butoxy radical. We conclude that there wassome involvement of the surface in the vapor phase coupling process overiron doped peroxides which lead to a steric constraint for 3°-3°coupling. Surface (barium oxide) ester intermediates are hypotheticalprecursors to such coupling products.

Although no carbon oxides are observed in stoichiometric pulseexperiments, spectroscopic examination of the surface of the iron-dopedperoxide reagents indicated an increase in carbonate carbon to bariumratio from which it appears that at least some carbon dioxide isinitially formed along with coupling and dehydrogenation products.

XPS measurements of a 10% Fe²⁺ /BaO₂ surface indicated that both +2 and+3 (possibly FeO₂) oxidation states of iron existed simultaneously onthe surface.

Transition metal compounds which may be used in the reagents for useaccording to the invention include compounds of copper, silver,titanium, zirconium, vanadium, chromium, molybdenum, tungsten, iron,cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium,platinum, lanthanum, cerium, praseodymium, gadolinium, dysprosium. Foroxidative dimerization, the Group VI metal, chromium, is not suitable,as shown in Run 11 in Table 1, but chromium is highly active fordehydrogenation, as shown in that run. For oxidative dimerization,transition metals from Groups I, III, IV, V, VII and VIII, andpreferably from Groups I and VIII, are used, while for dehydrogenation,transition metals from Groups I, III, IV, V, VI, VII and VIII, andpreferably from Groups I, VI and VIII, are used. Group IIB metalssuitable for use with barium peroxide in dehydrogenation of hydrocarbonsare zinc, cadmium and mercury.

The invention claimed is:
 1. Process for dehyrogenating branched-chainparaffins which comprises contacting a C₃ to C₄ hydrocarbon with areagent comprising barium peroxide oxidizer and a transition metal fromGroups I, III, IV, V, VI, VII or VIII of the Periodic Table or compoundthereof, in a reaction zone at a temperature sufficient to dehydrogenatesaid hydrocarbon, recovering a dehydrogenated hydrocarbon and bariumoxide from the reaction zone, and regenerating said oxidizer byoxidizing said barium oxide to barium peroxide.
 2. Process according toclaim 1 wherein said metal is iron.
 3. Process according to claim 1wherein said metal is chromium.
 4. Process according to claim 1 whereinsaid metal is cerium.
 5. Process according to claim 1 wherein said metalis silver.
 6. Process according to claim 1 wherein said regenerating isby heating the barium oxide in the presence of oxygen at a temperatureabove 400° C.